A gas phase polymerization process of polyolefin is usually conducted by a plurality of polymerization reactors which are serially and continuously connected, and polymer particles that have polymerized and grown in a reactor are then sequentially transferred to the next reactor as the polymerization proceeds, and finally particles having a uniform degree of polymerization are produced. As such a plurality of serially succeeding polymerization reactors, those consisting of gas phase polymerization reactors over the entire process, as well as those consisting of the first half of the process of the solvent polymerization method or bulk polymerization method using stirred tank reactors, or bulk polymerization method using loop reactors and the second half of the process using gas phase polymerization reactors have been known.
In a gas phase polymerization reactor, reactive olefin gas is circulated and forming a fluidized bed with polymer particles. Since the circulating gas is carrying fine polymer particles, the circulating gas is usually circulated after the fine polymer particles are separated the circulating gas by a cyclone disposed in the upper part of the reactor. However, the polymer fine particles may stay in the separating cyclone to agglomerate or come with the circulating gas into the circulating gas line and adhere to the inside of the piping, or stay on the inner surface of the heat exchanger to cause a clogging, or cause a clogging of the fluidization grid of the reactor. Once clogging occurs, it is necessary to stop the polymerization, dismount the circulating gas line and the like and remove the polymer clogging therein, resulting into the reduction in production amount and stopping of the reaction, and consequently, it is difficult to operate stably polymerization for a long time.
As an attempt to avoid such problems, Japanese Patent No JP 8-169915A (1996) discloses a cleaning method of adhered particles by sonic cleaning. However, it required enormous facilities to install sonic cleaners for every region where the fine particles of polymer coming with the circulating gas adhere. Japanese Patent Application No. JP 11-246608A (1999) discloses a method for preventing polymer powder from depositing in a circulating gas line by feeding a catalyst poison to the line, however, this method also caused another problem that, since the circulating gas containing the catalyst poison is fed continuously into the reactor, the circulating gas hindered the progress of the reaction. In addition, Japanese Patent Application No. JP 10-279612A (1998) discloses a method of reducing the amount of fine polymer particles coming with the circulating gas by designing the height of the fluidized bed as almost the same as the height of the polymerization reactor. However, in a multi stage polymerization, since the retention time, which is an important factor for determining the polymerization amount in the each reactor, is determined by the height of the fluidized bed, there was a problem that control of polymerization ratio of each reactor for controlling physical properties was restricted.